For building a power supply system, it is a significant challenge to expand a power transmission network more stably and, moreover, configure a system capable of introducing a large amount of natural energy. As a novel power network, a power network system called a digital grid (registered trademark) is proposed as disclosed in Japanese Patent Publication No. 4,783,453 and http://www.digitalgrid.org/ (digital grid consortium). A digital grid is a power network system in which a power network is divided into small-scale cells and the cells are asynchronously connected to one another. A divided power cell which is small has a scale including one house, building, or commercial facility. A divided power cell which is large has a scale including a prefecture, a city, a town, and a village. A power cell includes a load and, in some cases, a power-generating facility and a power storing facility. An example of the power-generating facility is a power-generating facility using natural energy such as solar power generation, wind power generation, and geothermal power generation.
To allow free generation of power in each of the power cells and smooth interchange of power among the power cells, the power cells are connected asynchronously. In other words, even when a plurality of power cells are connected to one another, the voltage, phase, frequency of power used in each power cell are not synchronized with those of another power cell. FIG. 28 is a diagram illustrating an example of a power network system 10. In FIG. 28, a utility grid 11 transmits base power from a large-scale power plant 12. A plurality of power cells 21 to 24 is set. Each of the power cells 21 to 24 has loads such as a house 31 and a building 32, power generating facilities 33 and 34, and a power storing facility 35. Examples of the power generating facilities are a solar power panel 33 and a wind power generator 34. The power storing facility is a storage battery 35 or the like. In the specification, the power generating facilities and power storing facilities will be also collectively called a “distributed power supply”.
Further, the power cells 21 to 24 have power routers 41 to 44, respectively, as connection ports to be connected to the other power cells or the utility grid 11. Each of the power routers 41 to 44 has a plurality of legs (LEG). In FIG. 28, the reference characters of the legs are not depicted and blank circles attached to the power routers 41 to 44 indicate connection terminals of the legs. The leg has a connection terminal and a power converter, and an address is assigned to each of the legs. A power conversion by a leg includes a conversion from alternating current to direct current or from direct current to alternating current, and a change of voltage, frequency, or phase of power.
The power routers 41 to 44 are connected to a management server 50 via a communication network 51 and are controlled integrally by the management server 50. For example, the management server 50 instructs the power routers 41 to 44 to transmit or receive power by the legs using the addresses assigned to the legs. By the operation, power interchange is performed among the power cells via the power routers 41 to 44.
By realizing power interchange among the power cells, for example, one power generating facility 33 or 34 and one power storing facility 35 can be commonly used by a plurality of power cells. When surplus power is interchanged among the power cells, while largely reducing the cost of the facilities, the power demand and supply balance can be stably maintained.
As described above, the management server 50 and the power routers 41 to 44 under the management server 50 communicate with each other via the communication network 51. In the case where the communication becomes interrupted due to a failure or the like, the management server 50 cannot control/monitor one or some of the power routers 41 to 44. When a power router is left without being controlled or monitored, a serious accident may be brought about. Therefore, it is desirable to safely detach the power router which became unable to communicate from the other power routers and power cells. The safe detachment is achieved by stopping the legs connected to the other power routers and other power cells, for the legs in the power routers which became unable to communicate.
However, the management server 50 cannot stop the legs in the power router which became unable to communicate directly via the communication network 51 for a reason that the communication to the power router is interrupted. There is also a method that a power router which became unable to communicate stops its legs by self-determination but the method should not be employed. The reason is that there may be other power routers and power cells depending on power transmitted from the power router which became unable to communicate. When the legs are stopped without considering the dependency relation, chain blackout may be caused.